The sub-specific
taxonomy of blue whales is not yet fully elucidated in all regions, but the
Antarctic blue whale is distinguished by its large body size and Antarctic distribution
in summer. It is morphologically distinct from its neighbouring subspecies, the
pygmy blue whale B. m. brevicauda
(Ichihara 1966), but only a limited number of the distinguishing
characteristics are discernible at sea, making field differentiation between
the two subspecies imperfect (Kato et al.
2002). The ranges of the two subspecies do not overlap in summer (Kato et al. 1995, Branch 2006) but they may
overlap in winter if the Antarctic blue whale migrates into pygmy blue whale
habitat. The morphological distinction between Antarctic blue whales and those
off western South America is not always clear (van Waerebeek et al. 1997, Palacios 1999) but the
summer presence of the southeastern Pacific whales and the gap in summer
sightings of blue whales between 44°S in Chile and the Antarctic implies that
the Antarctic whales are geographically well separated from the southeastern
Pacific whales. LeDuc et al. (2007)
found that Antarctic blue whales were genetically distinct at the population
level from pygmy blue whales and southeastern Pacific whales, but no
definitive, diagnostic genetic marker for the Antarctic blue whale has been
found to date. A recent acoustic study has shown that the sounds produced by
Antarctic and pygmy blue whales are distinct (Ljungblad et al. 1998). The unique songs produced by both forms are also
different from those produced by other blue whales world-wide (McDonald et al.2006).

Justification:
The population is less than 3% of its level of three generations ago (at least a 97% decline) (1914-2007), and therefore qualifies for CR under the reduction criterion A1 based on a combination of direct recent observations, and inferred reductions from known past catches (subcriteria a, b, d).

The Antarctic blue whale occurs in summer between the Antarctic Polar Front and into the pack ice zone, being most abundant near the ice edge (Branch et al. 2007). Modern sightings are aggregated close to the edge of the pack ice, while past catches extended further north. Whether this is due to retreat of the pack ice since the time of catching (de la Mare 1997), or because the distribution of the species has contracted following exploitation, is unclear. Over 40,000 blue whales were caught in the waters around South Georgia, but the species is rare there now (Moore et al. 1999).

The winter distribution is poorly known, except that Antarctic blue whales occur in winter around southern Africa (including South Africa, Namibia, Angola, and Congo) where they were formerly caught in large numbers (IWC 2006; Best 1998). Elsewhere there are scattered records, but the sub-specific identity is often in doubt. The assumption has been that animals migrate to lower latitudes in winter, but a portion of the population may remain in Antarctic all the year round.

The Antarctic blue whale
was extremely abundant in the past; about 341,830 blue whales were recorded
caught in the Antarctic and sub-Antarctic (IWC 2006) in the 20th
century, of which 12,618 were identified as pygmy blue whales or are assumed to
have been so from their location (Branch et
al. 2004). About 40,000 of these
were taken around South Georgia. In addition,
the majority of the over 17,000 blue whales caught off southern Africa were probably Antarctic blue whales (Branch et al. 2007). Ignoring these and other
catches north of 40°S, Branch et al.
(2004) estimated the pre-exploitation (1905) abundance to be 239,000
(202,000-311,000). Theestimated population size in 1996,
based primarily on data from the International
Decade of Cetacean Research (IDCR)
—later Southern Ocean Whale and Ecosystem Research (SOWER) programme - whale sightings cruises,
was 1,700 (860-2,900) with an estimated annual rate of increase of 7.3%
(1.4-11.6%). The default value of 31 years for generation time given in Taylor et al. (2007) was considered
appropriate, given an absence of any indications to the contrary from available
biological information for the species. That implies that the time window for
applying the A (reduction) criterion is 1914-2007. Branch et al.'s initial (1905) population estimate can be taken as a
conservative (i.e., negatively biased) proxy for the 1914 population size,
because few (< 1,000) catches were taken during 1904-11and those authors
excluded the 17,000 catches off Southern Africa.
Their estimate for the ratio of the 1996 population size to the initial is 0.7%
(0.3-1.3%). Even allowing for a doubling of the population size since 1996, the
Antarctic blue whale population remains below 3% of the 1914 level.

Antarctic blue
whales are the largest living animal, with lengths up to 33.6m. They feed
almost exclusively on euphausiids (krill), especially Euphausia superba (Mackintosh and Wheeler 1929). In summer they
feed predominantly near the edge of the pack ice zone.

The main threat in the
past was direct exploitation, which only became possible (on more than an
occasional basis) in the modern era using deck-mounted harpoon cannons. Whaling
on Antarctic blue whales began in 1904 based in South
Georgia. With the advent of factory-ship whaling in the 1920s,
catching spread around the Antarctic, reaching a peak in 1930-31 when more than
30,000 were taken. Southern Hemisphere
blue whales have been protected under the International Whaling Convention
since 1966. Continued illegal catches of blue whales by factory ships from the
former USSR
until 1972 were primarily pygmy blue whales. The Antarctic blue whale
population was reduced to a dangerously small size (low hundreds) by the end of
whaling but is now increasing (Branch et
al. 2004). Antarctic blue whales are not known to be subject to any current
direct anthropogenic threats.